1. Technical Field
This invention relates to the field of wire dot matrix print heads. More particularly, this invention relates to an improvement in print actuating means wherein multiple armatures for driving the print wires are constructed from a single piece of sheet metal, and a multifunctional constraint for all the armatures is provided by a one piece molded plastic cap which is uniformly spring loaded by a one piece spring and retaining ring.
2. Background Art
Wire dot matrix print heads have been in use for many years and are noted for their versatility and low cost. In the prior art, wire dot matrix print heads are known to include a circular arrangement of electromagnets which are selectively energized to attract a moving core mounted on an armature. The attraction of the moving core urges the armature against a print wire. The print wire is driven against a ribbon which leaves a single dot on a record medium such as paper. The accumulation of dots produces printed characters. Examples of dot matrix print heads are disclosed in U.S. Pat. Nos. 3,770,092; 3,892,175; 4,244,658 and 4,569,605.
In dot matrix print heads of the prior art, individual armatures or actuating levers are provided. The individual armatures are radially aligned about a center point with each armature extending inwardly to a tip. The tip of the armature engages the end of a print wire. Electromagnetic coils are positioned relative to the armatures so that energization current passing through the coils creates a magnetic field attracting the armature toward the coil. The armature, moving under the attractive influence of the magnetic field, drives the print wire toward a print position. Each of the armatures of the prior art functions as a cantilever in that the armature is supported at the base, along the periphery of the print head, by a leaf spring.
The objectives in the design of a print head are to combine high speed and efficiency with ease of manufacture. One way to achieve higher print speeds and efficiency is to minimize both the armature mass and the gap between the core and armature. As the armature mass decreases, speed and efficiency increase, especially when operating in the ballistic mode and core/armature gap decreases, but there is a concomitant increase in the difficulty of manufacturing the print head due to the smaller, lighter weight, more fragile components. This invention makes the print head easier to manufacture by combining all of the armatures and their individual return springs into one assembly, and also by combining all the constraining members into a one piece molded plastic cap. The result is a faster, more efficient print head that is easy to manufacture.